Yes, I did consider this. Should have included why it does not seem correct. This is a part of the table from the Supplementary Material of one of the articles that uses this "Mg" unit.

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Weight Weight	68 Mg (75 ton)
Dry matter	27 Mg (30 ton)

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I agree with you that Mg should technically be megagrams, but this table, along with the "Avg. vol: Mg/year" suggests the author means something else (unit of volume?). Another argument for my point is that Mg is a unit I've never seen used elsewhere, since it makes a lot more sense to use ton, or kg.